Deep fat fryers as commonly used commercially in, for example, fast food restaurants, must be capable of producing food of uniform quality, rapidly, and with an efficient utilization of fuel. In addition, such fryers must be constructed so that they can be easily serviced and cleaned.
Uniform heating conditions including a rapid response to a sudden drop in temperature have continued to be a problem in fryer design. As is well known, if the food to be cooked is frozen, when a basket is submerged in the heated oil, the oil will dramatically drop in temperature, and therefore excess heat will be required. Once the oil is returned to cooking temperature, the heat input must be controlled so that desired cooking conditions are not exceeded. It is desired, then, to design a deep fat fryer which will make maximum utilization of the heat input available so that when temperature conditions drop suddenly, the conditions can be returned to desired cooking conditions as rapidly as possible.
U.S Pat. No. 4,913,041 there is described a deep fat fryer which utilizes a dual gas fired burner assembly wherein hot combustion gases generated by the gas fired burners are permitted to flow through oil heating passages which are formed around suitable exterior surfaces of the fry pot well section or sections. As the hot combustion gas from the burner structure flows through metal heating passages, a significant portion of the available combustion gas heat is unavoidably diverted into the mass of metal used to form the heating passages in conjunction with which the fry pot walls are actually heated. Therefore, only a portion of the available gas heat is transferred inwardly. In that patent, then, it was intended to provide a fry pot with spaced well sections disposed on either side of a dual gas burner assembly. Exhaust gases from the burner assembly are discharged into a heating cavity before passing into an exhaust stack system, and insulation is also utilized to maximize the heat input to the oil. The depending well sections then are oil heating sections with the cooking pot being disposed above so that as heated oil rises, the cooler oil flows downward into the heating section. The burners described are ceramic radiant burners of conventional design.
In U.S. Pat. No. 5,050,582 cylindrical burners are used which are disposed in oblong tubes which extend through the oil tank and the exhaust gases are allowed to flow behind and under the lower portion of the tank before exiting through a flue. In flowing behind and under the bottom of the tank, a dual flow system is arranged with baffles so that maximum heat can be extracted from the exhaust gases. However, the combustion tubes extend through the lower portion of the tank and the recirculating flue gases are also disposed below and behind the tank whereby the portion of the tank exposed to the hot exhaust gases from a heat exchange standpoint is relatively small and remote from the cooking area. In addition, the heat source is described as a power burner in a combustion tube which is a relatively inefficient means for delivering heat to the tank.
One further apparatus for deep fat frying is described in U.S. Pat. No. 4,704,290. In this patent, the oil from the tank is circulated through an external heat exchanger in a pipe system and then returned to the cooking tank. In this way, the heat exchange can be controlled and the process does not depend upon convection currents for raising heated oil from the bottom of a tank to be replaced by cooled cooking oil from the top of the tank. In this instance, the oil is pumped through the heat exchanger and then returned to the upper portion of the tank. This procedure, of course, requires piping apparatus that must be cleaned to avoid fouling and, therefore, provides a maintenance problem in the system used to circulate the hot oil between the tank and the heat exchanger. A conventional burner is used as a heat source in the heat exchanger.
As is well known, heat from an open flame and a conventional boiler fire tube generally heats by convection currents. In an effort to increase the efficiency of such fire tubes, it was proposed to provide a ceramic shell or cylinder constructed of a porous matrix of ceramic fibers which would surround the flame. The purpose of such a construction would be to generate infra-red energy. See, for example, U.S. Pat. No. 4,519,770.
Ceramic fire tubes, however, are fragile and brittle and have been found to be difficult to maintain.
It was also described in U.S. Pat. No. 4,657,506 to provide a cylindrical metal member which is surrounded by a woven fabric of stainless steel. The fabric provides a porous surface so that a fuel air mixture admitted to the interior of the metal cylinder would pass outwardly through holes in the side thereof and through the metal fabric where it could be ignited. Such metal units, however, were relatively short lived because of oxidation and generally used a spark igniter. The spark igniter is subject to generation of unwanted electromagnetic energy would could interfere with adjacent instruments.
Although a porous ceramic matrix was insufficiently durable, and a porous stainless steel matrix oxidized too quickly, in U.S. Pat. No. 4,597,734 there was described an acceptable metal alloy, porous mesh burner. That burner in one embodiment was generally cylindrical having an axial plug. A fuel air mixture was released into the annular space between the plug and the matrix which would filter through the porous matrix to be ignited. The result was a radiant burner with the ability to withstand both oxidation and corrosive conditions as well as thermal shock. The alloy utilizes a yttrium component which was responsible for binding a coating of alumina formed on the metal surface to the metal. The alumina coating then provides resistance to high temperature corrosion and further oxidation. In the preferred embodiment, the mesh is centered for stability. Major ingredients are iron, chromium and aluminum, in addition to yttrium, silicon and carbon.